That isn't to say that I disagree with the format as a rule. It has its place, like everything else. But I think people can fall too easily into writing continuations instead of starting new ideas. We all love the characters and stories, but we don't always need to follow their whole lives. There is a proper place to end everything; if it drags on too much, I think we lose the ability to savor each moment. We think of quantity and let quality suffer.
Writers need to consider the question, what is the purpose of this book? Even if it is only written off of his own need, if it's published, it needs to be accomplishing something. Entertainment (then, what kind of entertainment?), a message, information. Just like writing a paper, everything needs to lead up to the greater purpose, including format. Like I said, entertainment can suffer if a book never ends. Or a message can be lost in copious pages.
I feel like a series ties me up too much. I have to read all the books, I have to keep track of when future books are coming out, I have to always be wanting more. I can't just read one book and savor it in my mind forever after. Reread it when I like, replay scenes in my head, discover new beauties. A series keeps going and going. Maybe it's reflective of our society: we always want more and forget to value what we already have.